Comment by lopsotronic

Current thought in the academy is that the flea-rat vector wasn't the dominant vector for the medieval plague "Black Death" mortality event, but human-flea-human. See also 2018 PNAS paper Katharine Dean, Nils Stenseth, Boris Schmid at Oslo. Basically, Northern Eurasian people were really gross, Mongols even moreso, and that provided the vector for a dominant human-flea-human spread. The rat vector, we're missing evidence that should be there: mass rat mortality; fast & huge mortality in areas with few rats; clustering followed human networks, not rodent networks. Also the kind of flea. Check the refs for more details.

The initial infection though, definitely from a central Eurasian rodent, likely a marmot somewhere near Kyrgyzstan.